It is known that natural gas condensates recovered from natural gas fields after eliminating liquefied petroleum gas and some crude oils contain mercury in amounts ranging from several tens to hundreds ppb by weight depending on their production districts. Mercury contained in such hydrocarbon oils is not in a single form but contained in the forms of elemental mercury, ionic mercury and organic mercury. When such liquid hydrocarbon oils are used as chemical feedstocks such as the feedstock of ethylene, mercury forms amalgam with palladium, platinum, copper, and aluminum and thus causes a hydrorefining catalyst to degrade. It is also known that the use of aluminum-based alloys as material for constructing an apparatus leads to decreased strength thereof due to amalgam corrosion.
Various methods for removing mercury contained in hydrocarbon oils have been studied and methods using various types of adsorbent have been proposed. Examples of methods for removing mercury with metal sulfides include a method using copper sulfide (Patent Literature 1) and a method using polysulfides of metals such as copper, nickel, iron, cobalt or the like (Patent Literature 2). There has been reported a method where liquid or gas containing mercury is brought into contact with an adsorbent containing a sulfide of one or more metal selected from the group consisting of molybdenum, tungsten and vanadium (Patent Literature 3). The methods described in these patent literatures provide high adsorptivity particularly to elemental mercury. However, in addition to elemental mercury, ionic mercury and organic mercury are contained in natural gas condensate and crude oil but cannot be removed with these above-described methods.
An example of a method for removing ionic mercury and organic mercury includes a method where organic mercury is decomposed in the presence of hydrogen using a catalyst and then removed with an adsorbent (Patent Literature 4). This method has, however, problems that a hydrogen plant is required under a circumstance with no hydrogen and some mercury is contained in hydrogen at the outlet of a reactor and discharged together therewith and thus mercury must be also removed from the discharged hydrogen. There is also a method where a liquid hydrocarbon oil containing hardly reactive mercury compounds (mercury dihalides such as mercury chloride, monoalkylmercury halides such as methylmercury chloride and dialkyl mercuries such as dimethyl mercury) is brought into contact with a metallic aluminum or a metallic zinc at a temperature of 200° C. or higher to decompose the hardly reactive mercury compounds and then the elemental mercury generated by the decomposition is removed by extraction using an extracting agent that is an aqueous solution containing an alkali polysulfide as a main component. This method does not need hydrogen for the decomposition and thus can overcome the above-described problems but cannot avoid the occurrence of considerable facility cost due to the need of an cracking unit and an extracting unit.
There is an alternative method wherein a hydrocarbon oil is brought into contact with an activated carbon or an activated carbon supporting a sulfurized alkali metal to remove mercury (Patent Literature 5). This method can remove mercury only with an adsorbing operation but the adsorbent described in this literature has a high adsorptivity to elemental mercury but is extremely weak in adsorptivity to ionic mercury and organic mercury and thus cannot treat a hydrocarbon oil containing ionic mercury and organic mercury constantly for a long period of time. Therefore, as described in Patent Literature 6, even when an activated carbon adsorbent is used, a hydrocarbon oil containing ionic mercury and organic mercury needs to be hydrotreated and then be brought into contact with an activated carbon adsorbent in order to treat such a hydrocarbon oil stably for a long period of time (Patent Literature 6).
In a technique to remove mercury from a hydrocarbon oil, it is more extremely difficult as described above to remove ionic mercury and organic mercury than to remove elemental mercury and thus a large facility cost has been required due to the necessity of an operation to decompose ionic mercury and organic mercury. Consequently, a method that can easily and efficiently remove ionic mercury and organic mercury has been demanded.